Since the practical use of the first antibiotic penicillin G in 1940, many antibacterial agents have been developed, and antibacterial chemotherapy has greatly contributed to the advance of modern medicine and the extension of the average lifetime. However, pathogenic bacteria have acquired resistance to such antibacterial agents one after another, and the effects of antibacterial chemotherapy have considerably decreased in the 21st century (see NPL 1). In particular, in 1960 in the United Kingdom, methicillin-resistant S. aureus (MRSA), which acquired resistance to all β-lactam antibacterial agents, appeared from Staphylococcus aureus (S. aureus) which is a pathogenic bacterium mainly causing hospital infection (see NPL 2). Since then, the number of MRSA has continued to increase up to the present, so that it has spread all over the world today. It is not too much to say that there are no hospitals without MRSA.
There are many types of antibacterial agents. Thus, even when one type of antibacterial agent is ineffective to an infectious disease, the infectious disease can be cured using another type of antibacterial agent having a different action mechanism. Pathogenic bacteria, however, have continued to acquire resistance to each of the antibacterial agents. As a result, by the end of the 20th century, multiply antibiotic-resistant bacteria appeared which acquired resistance to almost all currently-available antibacterial agents. Typical examples of the multiply antibiotic-resistant bacteria include MRSA as Gram-positive bacteria, Acinetobacter as Gram-negative bacteria, and tuberculosis bacteria as acid-fast bacteria.
Vancomycin was an only antibacterial agent that maintained its effectiveness to multiply antibiotic-resistant bacteria. However, there has currently appeared vancomycin-resistant Enterococcus (VRE) and the like that acquired drug resistance to vancomycin, which needs sufficient care to be taken in using vancomycin.
Therefore, keen demand has arisen for the provision of a novel compound which exhibits a low antibacterial activity against bacteria acquiring no drug resistance and thus does not permit them to become drug resistant and which exhibits an excellent antibacterial activity against drug-resistant bacteria resistant to at least one drug such as vancomycin (see NPL 3).
Also, there have recently been problems with infectious diseases caused by bacteria belonging to the genus Clostridium, which are pathogenic enterobacteria. Bacteria belonging to the genus Clostridium have characteristics that they are positive in Gram staining, obligately anaerobic, and form spores. Some of the bacteria belonging to the genus Clostridium are present as normal bacterial flora within the intestine of a human and animals, and pathogenic bacteria are also present there. Among them, there have been problems with infectious diseases such as colitis caused by Clostridium difficile and Clostridium perfringens. 
More than 400 kinds of enterobacteria reside in the intestine of a healthy human in a well-balanced manner. When an antibacterial agent is administered against the intestinal flora (intestinal normal bacterial flora) formed by these enterobacteria, the balance of the intestinal flora is broken, so that Clostridium difficile originally existing in the intestine proliferate abnormally due to microbial substitution to produce toxin in the large intestine. Also, Clostridium difficile is propagated from the developed patients directly or via medical staff in hospitals. Abnormal proliferation of Clostridium difficile not only causes severe diarrhea in patients but also often causes them to die. 20% to 30% of the cases of diarrhea and 90% of the cases of pseudomembranous enterocolitis accompanied by use of antibacterial agents for patients in hospitals are due to abnormal proliferation of Clostridium difficile. Infectious diseases caused by Clostridium difficile are characterized by being high in morbidity rate and death rate.
In many advanced countries such as the United States of America, Canada, and Europe, hospital infection caused by Clostridium difficile has become serious problems. For example, only in the United States of America, about 700,000 cases occur in a year, and economic impact relating to this infectious disease is as much as 3.2 billion dollars per year. In member states of the EU, medical care cost relating to it is estimated to be 4.4 billion dollars in a year. In Japan, although infection of Clostridium difficile is not usually tested, it is considered that there are many potentially infected individuals.
Almost all antibacterial agents do not effectiveness to Clostridium difficile and in that sense, Clostridium difficile can also be said to be one of the multiply antibiotic-resistant bacteria. There are only vancomycin and metronidazole used today as therapeutic drugs against infectious diseases caused by Clostridium difficile, and there has been a serious problem with reoccurrence upon withdrawal of the drugs. Drug resistance thereto will become a problem eventually, and at present it has strongly been desired to develop a novel compound having novel antibacterial activity against Clostridium difficile. 